Unlockables
Unlocking Remilia, Yuyuko, Yukari, and Suika for Arcade, Duel CPU, Duel Human, and Practice Modes
To unlock any of these 4 for all non-Story Modes, simply encounter and defeat them in Story Mode. (Unsure of whether Arcade Mode works as well)
Unlocking Remilia and Yuyuko for Story Mode
To unlock Remilia and Yuyuko for Story Mode, complete Story Mode for the first 6 characters.
Quirks
Not sure if this is a bug or intended, but it's definitely something neat. The song 幽境 (Track 29 in sound test), actually played all of the music pieces in the game's version. First noticed something odd when it switched pieces in the demo before fighting Marisa. It only cycles through the few tracks within the demo, if you try it from the demo, but in the full game, it's almost like a trial of all the in-game music!